Local and landscape factors in differently managed arable fields affect the insect herbivore community of a non-crop plant species.
The expansion of simplified ecosystems such as intensively managed annual crops plays a big part in driving the global biodiversity crisis. Field-scale diversification, for example leaving weeds to grow in crops, is one way in which diversity in agro-ecosystems can be restored. However, little is known about the determinants of the non-crop plant-based insect communities within arable fields at local and larger spatial scales, an essential component in extrapolating plant diversity benefits to higher trophic levels. We investigated how diversification of agro-ecosystems at the field and landscape levels affects the insect community of the creeping thistle Cirsium arvense. Artificial plots of the host-plant were established in three regions of Germany in 48 paired organic (diverse, weeds not controlled with herbicides) and conventional (simplified, very low weed density and species richness) wheat fields across a gradient of landscape heterogeneity, from simple arable-dominated to heterogeneous, diverse landscapes. Leaf-feeding herbivores were monitored directly, while stem-boring herbivores and their parasitoids were quantified by dissecting the stems of the thistles. Land-use types and naturally occurring thistle stands were mapped within a radius of 1 km around each thistle plot. Herbivore species richness was enhanced by both organic farming and landscape heterogeneity but not by higher densities of thistles in the landscape. For most of the species, host-plant plots in organic fields were more likely to be colonized than those in the conventional fields. The enhancement of diversity in organic fields is probably the result of a slightly higher natural cover of the host-plant Cirsium arvense. Synthesis and applications. Both diversification of landscape (fewer arable crops, more perennial habitats) and extensification through organic management are effective measures of enhancing arthropod diversity on weeds. The impact of field-scale agri-environment schemes on biodiversity should be supplemented by including landscape-scale diversification programmes to include a minimum level of perennial habitat cover. Biodiversity benefits of organic agriculture rely for a large part on non-crop plants. Weed populations should be allowed to coexist with the crop to maintain these benefits, which are threatened by more intensive 'organic' management, such as heavy mechanical weed control.